A sequence of bentonite and shale samples in a gouge zone of the Lewis Thrust (Alberta, Canada) that display increasing degree of transformation of clay minerals toward the hanging wall of the thrust has been studied by X-ray diffraction (XRD), X-ray texture goniometry (XTG), scanning electron microscopy (SEM), and transmission and analytical electron microscopy (TEM-AEM), to examine the relations among mineral transformations, microfabrics and fault zone properties. TEM images of authigenic clays show abundant smectite in shale away from the hanging wall, characterized by anastomosing layers with an average orientation that parallels bedding, coexisting with uncommon R1 illite-smectite (I-S). In the sample nearest the hanging wall, by contrast, the dominant clay is mixed-layered, illite-rich ilite-smectite (R1 I-S), coexisting with discrete illite, occurring in individual packets of relatively straight layers with well-defined boundaries. Deformed clay packets are common. Pore space, where packets intesect at high angles to one another and to bedding, is abundant (c. 25%). The microfabric and proportion of illite of intermediate samples are transitional to these end-members. Inter-layered bentonite samples show properties that are similar to those of shale. TEM observations are supported by quantification of the fabrics using XTG, which shows that the intensity of clay preferred orientation decreases significantly with increasing illitization. These relations imply that faulting was the cause of mineral transformations and formation of secondary pore space. The illitization reaction rate was enhanced both by stress-induced defects in clays, and by increased water/rock ratio resulting from deformation-related pore space, resulting in lowering of the effective stress. The deformation-enhanced reaction thus created a positive feedback for further faulting in clay gouge, leading to enhanced weakening of the fault zone.

The normal prograde diagenetic and low-grade metamorphic sequence of dioctahedral clay minerals including illite-rich I-S and illite, as observed by TEM, proceeds from a partially disordered 1Md stacking sequence to 2M1; i.e. 1M does not normally occur as an intermediate polytype. Examples of 1M illite stacking sequences have been studied, however, from the Golden Cross gold deposit, New Zealand, the Broadlands-Ohaaki geothermal system, New Zealand, the Potsdam Sandstone, New York, and the Silverton Caldera, Colorado. Specific clay-mineral packets identified by TEM techniques as 1M illite were found to have anomalously high Mg contents. The Broadlands illite provides the most definitive data, as separate packets of 1M and 2M1 illite coexist. Average compositions for 1M and 2M1 illite are (K1.66Ca0.04)Σ1.70(Al3.32Fe0.31Mg0.57Mn0.06)Σ4.26(Si6.43Al1.57)Σ8O20(OH)4 and (K1.57Na0.31Ca0.03)Σ1.91(Al3.58Fe0.05Mg0.29Mn0.01)Σ3.93(Si6.70Al1.30)Σ8O20(OH)4, respectively. In addition, 1Mdillite, which is the polytype occurring in the common 1Mdto 2M1 prograde sequence, is relatively Mg poor, but coexists with Mg-rich illite in the Silverton Caldera sample.These data confirm that 1M stacking is caused by compositional anomalies, and thus explain the lack of the 1M stacking sequence in normal diagenetic sequences in pelitic rocks, as most illite in such environments has a relatively small phengitic component. The parameter Δz, a measure of the corrugation of the oxygen sheets, may be the key parameter reflecting the polytypic state of dioctahedral and trioctahedral micaceous minerals. Such composition-determined relations may be related to the occurrence of 1M polytypism in glauconite and celadonite, both dioctahedral 2:1 clay minerals having large Mg or Fe octahedral-cation components, and in trioctahedral micas. Insofar as the 1M stacking sequence does not have the same composition as 2M1 material, these data confirm that the different varieties of illite are not polytypes, sensu stricto.

Abstract--The clay mineral textures, assemblages, formation mechanisms, and controlling geological parameters relating to alteration of silicic volcanic rocks by hydrothermal solutions, in core samples from the Broadlands-Ohaaki hydrothermal system, New Zealand, were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission and analytical electron microscopy (TEM/ AEM). Mineralogical and textural relations of this active hydrothermal system, for which temperatures and fluid relations are well known, are equivalent to those in the Golden Cross hydrothermal gold deposit as described in Part 1.XRD data show a sequence of clay minerals from smectite to a range of interstratified I-S to mica with increasing depth and temperature, on average. TEM observations are in general agreement with XRD data, especially with respect to relative proportions of illite (I)-and smectite (S)-like layers. TEM data also show that: (1) Smectite packets contain no discrete illite-like layers in samples identified as (Reichweite, R = 0) I-S by XRD. They coexist with separate packets of (R = 1) I~S. (2) A continuous range in I-S occurs from (R = 1) I-S with increasing proportion of illite-like layers, but at high illite-like layer contents there is a gap between I-S and illite. (3) 1M and 2M] polytypes of mica coexist in separate packets, but the rare 1M polytype has a larger VlMg content.The data imply that clay minerals formed by dissolution and neocrystallization directly from volcanic phases, although multiple reaction events can not be ruled out. Such "episodic" alteration produces a sequence of clay minerals identical to those of prograde diagenesis of pelitic sediments. This result implies that the presence of a continuous sequence is not definitive proof of continuous sequences of transformation as a function of time and continuous burial. Reaction progress of the clay-mineral sequence is in general accord with the known temperature gradient, but with significant and common exceptions. High porosity and permeability, both inherent in rock texture and local structure, are inferred to fbster local reaction progress, as consistent with metastability of phases and the Ostwald step rule.

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